Obesity increases risk for diabetes and atherosclerotic cardiovascular disease (CVD). However, the underlying mechanisms remain poorly understood. It is currently known that obesity is associated with chronic inflammation-characterized by increased production and secretion of cytokines/chemokines by adipose tissue (AT) and liver-and with increased hepatic production of triglyceride-rich lipoproteins (TGRLs) and increased levels of fatty acids (FAs, saturated FAs in particular). Current data suggest that these changes activate leukocytes in blood and AT. Blood monocyte activation can increase monocyte adhesion and migration, which is implicated in the development of atherosclerosis and accelerates inflammation in AT. Leukocyte accumulation and activation in AT may be critical to the development of adverse metabolic and pathogenic effects of obesity. CD11c is a b2 integrin primarily expressed on monocytes/macrophages and dendritic cells (DCs) that is a marker for monocyte/macrophage activation and participates in monocyte recruitment. Our preliminary data show that: (1) obesity with insulin resistance induced by high-fat (HF) diet rich in saturated FAs up-regulates CD11c on blood monocytes and AT macrophages; (2) CD11c contributes to monocyte adhesion and migration on inflamed endothelial cells (ECs) by binding vascular cell adhesion molecule-1 (VCAM-1); and (3) deficiency of CD11c in obese mice reduces atherosclerosis and decreases AT inflammation. Therefore, we hypothesize that obesity is associated with monocyte activation with increased CD11c expression, and that CD11c is mechanistically linked to obesity-related diseases including CVD and diabetes. Three specific aims are proposed to test our hypotheses: Aim 1. Examine blood monocyte activation, including CD11c expression, in obese mice and humans; and its modulation by weight loss and by dietary eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA); Aim 2. Determine the role of increased CD11c in the development of atherosclerosis and in monocyte adhesion and recruitment into atherosclerotic lesions by using animal models of obesity with hypercholesterolemia, and by in vitro flow adhesion assay and in vivo adoptive transfer of blood monocytes with or without CD11c; and Aim 3. Examine the role of CD11c in AT inflammation including macrophage and T cell accumulation and activation; and in metabolic abnormalities in mice with HF diet-induced obesity. This approach will help us better understand the mechanisms of obesity- linked inflammation, specifically the role of CD11c in recruitment and activation of macrophages and T cells in atherosclerotic lesions and AT. Considering the demonstrated role of chronic inflammation in obesity-related CVD and metabolic disease, this approach is of significant interest and has the potential to provide novel preventive and therapeutic targets for obesity-linked diseases.